CN111207837A - Four-point calibration method for foundation microwave radiometer based on waveguide switch - Google Patents
Four-point calibration method for foundation microwave radiometer based on waveguide switch Download PDFInfo
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Abstract
The invention discloses a four-point calibration method of a foundation microwave radiometer based on a waveguide switch, which comprises a four-point calibration composition, calibration operation realization and a real-time internal calibration process during normal work. The method inserts a waveguide switch structure at the rear end of a feed source, under the condition that the direction of an antenna is not changed, a received data period is controlled through a time sequence, waveguide switch switching and a noise source are started to form two-point internal calibration self-circulation operation, and the cold calibration source and the heat calibration source of the antenna opening surface are cooperated with two points of a cold source and a heat source of the antenna opening surface through the rotation of a motor, so that a cold calibration source and a heat calibration source of the antenna opening surface and a normal temperature load and noise source four-point calibration system at the rear end of the antenna feed are.
Description
Technical Field
The invention relates to the field of foundation microwave remote sensing equipment, in particular to a four-point calibration method for a foundation microwave radiometer based on a waveguide switch.
Background
The ground microwave radiometer is ground microwave remote sensing equipment which utilizes microwave signals of atmospheric radiation transmitted from various heights to judge the change of atmospheric temperature and humidity, and has an important detection function on medium and small scale weather phenomena, such as storm, lightning, heavy rainfall, fog, freezing and boundary layer turbulence. The research of temperature, humidity and liquid water of a troposphere section, weather and climate models of a foundation microwave radiometer, satellite tracking (GPS, Galileo) wet/dry delay and humidity profiles, near forecast atmospheric stability (disastrous weather detection), temperature inversion detection, fog and air pollution, absolute calibration of a cloud radar and wet/dry delay correction VLBI have important application and become an important detection means outside a sounding balloon and a weather radar. The ground-based microwave radiometer has independent working capacity, can work under almost various environmental conditions, is suitable for an automatic weather station, is commonly used for inverting a complete atmospheric profile, and completely stores inverted data and original data, and can provide a complete custom or global standard algorithm.
The ground microwave radiometer is used in ground environment and needs to be unattended for a long time, the dynamic drift of the receiving system is always an important factor disturbing the reliability of the received data, and real-time calibration is the key for solving the problem. Compared with an antenna-based radiometer system, the standard cold air-external calibration source of the antenna aperture surface cannot be realized during remote sensing observation of the ground-based radiometer, so that real-time calibration from the antenna aperture surface is difficult, and the real-time calibration can only be realized from an internal calibration method at the rear end of an antenna feeder.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention provides a four-point calibration method for a foundation microwave radiometer based on a waveguide switch.
In order to achieve the above purpose, the technical solution for solving the technical problem is as follows:
a foundation microwave radiometer four-point calibration method based on a waveguide switch specifically comprises the following steps:
step 1: four-point calibration;
step 2: the calibration operation is realized;
and step 3: and (4) real-time internal calibration during normal work.
Further, the four-point calibration structure described in step 1 includes an antenna feed system composed of an antenna reflector-polarization separation device-K, V waveband feed source, a pitching scanning mechanism with a stepping motor driving a main reflector to rotate, a receiving front end composed of a waveguide switch and directional coupling, a four-point calibration source composed of a liquid nitrogen cold calibration source, a hot calibration source working at normal temperature, a load working at normal temperature and a noise source, and a receiver.
Further, the scaling operation in step 2 is implemented, and specifically includes the following steps:
step 21: conventional black body external calibration, i.e., thermal calibration TH: an upper computer inside the foundation radiometer executes a program to guide the antenna pitching motor to point right below, and the main antenna aperture surface wave beam just shines into the thermal calibration source black body;
the control signal K1 makes the waveguide switch connect the feed source, the heat calibration source signal is transmitted to the directional coupler at the rear end through the antenna reflection surface and polarization separation, and finally transmitted to the radiometer receiver system, in addition, the control signal K2 does not excite the noise source to work at low level, and the detection voltage VH and the normal temperature TH of the corresponding receiver are directly read;
step 22: cold source external scaling, i.e. cold scaling source TL: the prepared external calibration source is placed above an antenna housing of the foundation radiometer, an internal upper computer executes a program to guide an antenna pitching motor to point right above, and the main antenna aperture surface wave beam just shines into a cold calibration source black body;
the same as step 21, the control signal K1 makes the waveguide switch connect the feed source, the cold source signal is transmitted to the directional coupler at the rear end through the antenna reflection surface and polarization separation, and finally transmitted to the radiometer receiver system, in addition, the control signal K2 low level does not excite the noise source to work, and the corresponding receiver detection voltage VL is directly read;
TL is calculated as follows:
TL=T0-0.00825×(1013.25-P0)
wherein T0 is the environmental temperature at calibration time and has a unit of K, and P0 is the atmospheric pressure at calibration time and has a unit of Pa;
determining a K factor and a scaling equation through the external scaling operation of the steps 21 and 22:
T=K×V+b
wherein T is the brightness temperature, K is a scale factor, and b is a constant factor;
step 23: external calibration of noise source, namely noise source TN: the noise source is controlled to be started through a K2 switch, and receiver voltage values Vnh and Vnl in the external calibration of the cold source and the heat source are respectively read, so that the equivalent bright temperature of the mouth surface of the TN is determined:
step 24: load source calibration, namely normal-temperature load TR: the signal of the antenna feed surface is cut off through K1 switch control, the antenna feed surface is connected with a broadband waveguide load, the noise source is controlled to be started through K2, and the voltage values Vr and Vnr of the receiver are respectively read, so that the equivalent brightness temperature of the mouth surface of the TR is determined:
and completing the four-point calibration operation through the four steps of operation.
Further, in step 22, the voltage signal output by the receiver during microwave remote sensing and the actual target remote sensing brightness temperature bear a linear relationship:
T＝K×V+b
wherein T is the bright temperature, K is a scale factor, b is a constant factor, and the accuracy of the T is determined by the accuracy of the target remote sensing bright temperature data;
the ground microwave radiometer equipment is highly accurate sensitive equipment, the sensitivity of the ground microwave radiometer equipment can reach below 1K, any small fluctuation can cause inaccuracy of measured data, and therefore errors and misjudgments are caused for subsequent inversion. The influence on the data accuracy can be obtained from the following three formulas:
b is bandwidth, tau is integration time, TS is system equivalent noise temperature, G0 is system gain, ▽ Ts is system inherent sensitivity, ▽ Tg is uncertainty caused by system gain fluctuation, and ▽ T is system sensitivity;
the first formula reflects the inherent sensitivity of the system, is determined by the overall noise temperature Ts of the system, and can calculate and verify through the mean square difference value of the sampling voltage while scaling and extracting the K factor outside the cold source and the heat source:
▽Ts=K×▽V
wherein ▽ v is the most sampled voltage mean square error;
the second term reflects the uncertainty caused by the system gain fluctuation, and the root mean square values of the second term and the second term reflect the data drift of the whole machine system, namely the system sensitivity.
Further, the real-time internal calibration in normal operation in step 3 specifically includes the following steps:
step 31: factor correction during normal operation
In a normal working state, the antenna points to a zenith to receive an atmospheric radiation signal or a boundary layer mode and a tracking satellite mode, the antenna port surface does not carry out external calibration operation, but in order to ensure the accuracy in a data period, the internal calibration correction in one data period is realized by the waveguide switch and the noise source time sequence control:
and substituting the correction factors into the scaling equation to determine a new scaling equation: t ═ K × V + b, thereby implementing an internal scaling process;
step 32: noise source equivalent correction at gap time interval
The stability correction of an internal noise source is realized in a single-point external calibration mode, at a gap observed by remote sensing or at a boundary layer observation starting point, a program control program guides an antenna pitching motor to point to the right lower side, and a main antenna aperture surface wave beam just irradiates a thermal calibration source black body;
and controlling the noise source to be started through a K2 switch, and respectively reading voltage values Vnh and Vh of the noise receiver under the external calibration of the heat source, so as to determine the equivalent brightness temperature correction of the oral surface of TN:
Tn*＝K×(Vnh-Vh)。
preferably, the four-point calibration operation described in step 2 uses half a year as a cycle unit, the factor correction in normal operation described in step 31 in step 3 uses seconds as a cycle unit, and the noise source equivalent correction in the gap period described in step 32 in step 3 uses half an hour as a cycle unit, so as to ensure the calibration accuracy and the reliability of the remote sensing data from long, short and medium periods.
Due to the adoption of the technical scheme, compared with the prior art, the invention has the following advantages and positive effects:
the method inserts a waveguide switch structure at the rear end of a feed source, and cooperates with a cold source and a heat source of an antenna aperture surface through the rotation of a motor, so as to form a cold calibration source and a heat calibration source of the antenna aperture surface and a normal-temperature load and noise source four-point calibration system at the rear end of the antenna feed, wherein the four-point calibration operation generally takes half a year as a period unit. Under various conditions of antenna pointing direction invariance (fixed point remote sensing), satellite tracking and boundary layer scanning, the remote sensing scaling factor is corrected in real time by forming two-point internal scaling self-circulation operation through time sequence control, waveguide switch switching and noise source starting in each remote sensing receiving data period, and the accuracy of scaling and the reliability of remote sensing data are guaranteed by generally taking seconds as a period unit.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. It is obvious that the drawings in the following description are only some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive effort. In the drawings:
FIG. 1 is a diagram of two operating states of a waveguide switch and the relationship between the operating states and control signals according to the present invention;
FIG. 2 is a schematic diagram of a conventional external black body calibration (thermal calibration TH) implementation of the present invention;
FIG. 3 is a schematic diagram of a cold source external scaling (cold scaling source TL) implementation of the present invention;
FIG. 4 is a schematic diagram of a noise source external calibration (noise source TN) implementation of the present invention;
FIG. 5 is a schematic diagram of a normal remote sensing state internal calibration implementation of the present invention;
FIG. 6 is a schematic diagram of calibration switch timing control in a normal remote sensing state in accordance with the present invention;
FIG. 7 is a schematic diagram of an implementation of timely correction of a noise source TN according to the present invention.
Detailed Description
While the embodiments of the present invention will be described and illustrated in detail with reference to the accompanying drawings, it is to be understood that the invention is not limited to the specific embodiments disclosed, but is intended to cover various modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.
The embodiment discloses a four-point calibration method for a foundation microwave radiometer based on a waveguide switch, which specifically comprises the following steps:
step 1: four-point calibration;
step 2: the calibration operation is realized;
and step 3: and (4) real-time internal calibration during normal work.
Specifically, the four-point calibration structure in step 1 includes an antenna feed system composed of an antenna reflector-polarization separation device-K, V waveband feed source, a pitching scanning mechanism with a stepping motor driving a main reflector to rotate, a receiving front end composed of a waveguide switch and directional coupling, a four-point calibration source composed of a liquid nitrogen cold calibration source, a hot calibration source working at normal temperature, a load working at normal temperature and a noise source, and a receiver.
Specifically, the scaling operation implementation described in step 2 specifically includes the following steps:
step 21: conventional black body external calibration, i.e., thermal calibration TH: an upper computer inside the foundation radiometer executes a program to guide the antenna pitching motor to point right below, and the main antenna aperture surface wave beam just shines into the thermal calibration source black body;
the control signal K1 makes the waveguide switch connect the feed source, the heat calibration source signal is transmitted to the directional coupler at the rear end through the antenna reflection surface and polarization separation, and finally transmitted to the radiometer receiver system, in addition, the control signal K2 does not excite the noise source to work at low level, and the detection voltage VH and the normal temperature TH of the corresponding receiver are directly read;
step 22: cold source external scaling, i.e. cold scaling source TL: the prepared external calibration source is placed above an antenna housing of the foundation radiometer, an internal upper computer executes a program to guide an antenna pitching motor to point right above, and the main antenna aperture surface wave beam just shines into a cold calibration source black body;
the same as step 21, the control signal K1 makes the waveguide switch connect the feed source, the cold source signal is transmitted to the directional coupler at the rear end through the antenna reflection surface and polarization separation, and finally transmitted to the radiometer receiver system, in addition, the control signal K2 low level does not excite the noise source to work, and the corresponding receiver detection voltage VL is directly read;
TL is calculated as follows:
TL＝T0-0.00825×(1013.25-P0)
wherein T0 is the environmental temperature at calibration time and has a unit of K, and P0 is the atmospheric pressure at calibration time and has a unit of Pa;
the scaling operation of the method essentially determines the coefficient factors in the scaling equation and determines the oral surface equivalence of the internal scaling source. The calibration method comprises the steps of randomly carrying a heat calibration source TH in a microwave radiometer, finishing calibration operations of delivery, startup and applicable period through cooperative matching with an external cold calibration source, and determining coefficient factors of the calibration operation through a corresponding formula. Specifically, the K factor and the scaling equation are determined through the external scaling operation in steps 21 and 22:
T＝K×V+b
wherein T is the brightness temperature, K is a scale factor, and b is a constant factor;
step 23: external calibration of noise source, namely noise source TN: the noise source is controlled to be started through a K2 switch, and receiver voltage values Vnh and Vnl in the external calibration of the cold source and the heat source are respectively read, so that the equivalent bright temperature of the mouth surface of the TN is determined:
step 24: load source calibration, namely normal-temperature load TR: the signal of the antenna feed surface is cut off through K1 switch control, the antenna feed surface is connected with a broadband waveguide load, the noise source is controlled to be started through K2, and the voltage values Vr and Vnr of the receiver are respectively read, so that the equivalent brightness temperature of the mouth surface of the TR is determined:
K. b, TN and TR form the basic unit of the calibration file, and as TR is a completely passive device, the value of TR is only changed by the temperature of the environment where TR is located, but the constant temperature operation of the foundation microwave radiometer can determine that TR is always constant and TN is relatively constant.
And completing the four-point calibration operation through the four steps of operation.
Further, in step 22, the voltage signal output by the receiver during microwave remote sensing and the actual target remote sensing brightness temperature bear a linear relationship:
T＝K×V+b
wherein T is the bright temperature, K is a scale factor, b is a constant factor, and the accuracy of the T is determined by the accuracy of the target remote sensing bright temperature data;
the ground microwave radiometer equipment is highly accurate sensitive equipment, the sensitivity of the ground microwave radiometer equipment can reach below 1K, any small fluctuation can cause inaccuracy of measured data, and therefore errors and misjudgments are caused for subsequent inversion. The influence on the data accuracy can be obtained from the following three formulas:
b is bandwidth, tau is integration time, TS is system equivalent noise temperature, G0 is system gain, ▽ Ts is system inherent sensitivity, ▽ Tg is uncertainty caused by system gain fluctuation, and ▽ T is system sensitivity;
the first formula reflects the inherent sensitivity of the system, is determined by the overall noise temperature Ts of the system, and can calculate and verify through the mean square difference value of the sampling voltage while scaling and extracting the K factor outside the cold source and the heat source:
▽Ts＝K×▽V
wherein ▽ V is the mean square error of the most sampled voltage;
the second term reflects the uncertainty caused by the system gain fluctuation, and the root mean square values of the second term and the second term reflect the data drift of the whole machine system, namely the system sensitivity.
The first inherent characteristic is fixed at the design stage depending on the level of the selected device and the design method of the receiver system link. The key of the long-term stability is to ensure that the second term is as small as possible, and the long-term stability can be realized through a real-time calibration and constant-temperature working mode. However, the ground-based radiometer product is different from the space-based radiometer product, and cannot acquire the calibration source reference of cold air. A double-switch (waveguide switch and PIN switch (control noise source opening) structure is adopted, real-time normal-temperature load and high-temperature noise source two-point internal calibration under scene isolation is realized, so that the influence of receiving access gain fluctuation on telemetering data is eliminated, and the reliability of temperature, humidity and atmosphere telemetering data of a foundation microwave radiometer under long-term unattended operation is ensured.
Specifically, the real-time internal calibration during normal operation in step 3 specifically includes the following steps:
step 31: factor correction during normal operation (i.e. real-time internal calibration during normal operation is essentially a process of timely minor correction of coefficient parameters in a calibration file)
In a normal working state, the antenna points to a zenith to receive an atmospheric radiation signal or a boundary layer mode and a tracking satellite mode, the antenna port surface does not carry out external calibration operation, but in order to ensure the accuracy in a data period, the internal calibration correction in one data period is realized by the waveguide switch and the noise source time sequence control:
thus, a new calibration file is formed by K, b, TN and TR, and the iteration is replaced. And substituting the correction factors into the scaling equation to determine a new scaling equation: t ═ K × V + b, thereby implementing an internal scaling process;
step 32: noise source equivalent correction at gap time interval
The stability correction of an internal noise source is realized in a single-point external calibration mode, at a gap observed by remote sensing or at a boundary layer observation starting point, a program control program guides an antenna pitching motor to point to the right lower side, and a main antenna aperture surface wave beam just irradiates a thermal calibration source black body;
the noise source is controlled to be started through a K2 switch, and the voltage values Vnh and Vh of the noise receiver are respectively read under the external calibration of the heat source, so that the equivalent brightness temperature correction of the interface surface of the TN is determined, although the TN works at constant temperature, the TN is formed by semiconductor active devices after all, long-term stability drift exists, and the correction is carried out through the periodic single-point external calibration cooperation:
Tn*＝K×(Vnh-Vh)
preferably, the four-point calibration operation described in step 2 is performed by taking half a year as a cycle unit, the factor correction in normal operation described in step 31 in step 3 is performed by taking seconds as a cycle unit, and the noise source equivalent correction in the gap period described in step 32 in step 3 is performed by taking half an hour as a cycle unit, so that the calibration accuracy and the reliability of the remote sensing data are ensured from each of the long, short and medium periods through a series of control and operation.
Example 1: conventional radiometer calibration operation (waveguide switch is in working state 1 (as shown in fig. 1 below), 1 (feed source), 2 (directional coupler) ports, 3 (load) and 4 (null) ports are communicated, TTL level of K1 is low, noise source is in low level closed state)
In principle, system calibration is required before any field of ground microwave radiometer is moved and started, and the basic requirement of the system calibration is to determine the corresponding relation of converting 0-level voltage data into 1-level brightness temperature data. In general, radiometer receivers are designed with a linearity requirement of 99.9%, so that a cold-hot two-point external calibration determines a calibration equation, T ═ K × V + b
The method comprises the steps of placing a calibration source (box) accessory carried at random together with a frame at the upper end of a machine antenna housing at a fixed position, injecting liquid nitrogen into the box, immersing a calibration source black body above the liquid nitrogen, and closing an upper cover of the box (the liquid nitrogen is a dangerous article, and the liquid nitrogen is poured into the box and should be operated by a professional and burnt carefully).
The pitching stepping motor of the ground-based microwave radiometer is set (100000 steps/one week, antenna aperture surface points to 0 point) to return to the original point, so that the antenna aperture surface points to the blackbody source inside the machine, as shown in fig. 2, the output voltage value VH when stable is read, and the internal blackbody temperature TH is read.
The step motor of the ground microwave radiometer rotates 50000 steps, the random outer zone directly above the antenna opening face points to soak the liquid nitrogen calibration source, as shown in fig. 3, the output voltage value VL when the stability is read, the ambient temperature T0 and the earth surface atmospheric pressure are read, and the ambient temperature T0 and the earth surface atmospheric pressure are converted into the cold source black body temperature TL.
TL＝T0-0.00825×(1013.25-P0)
From the tested VH, VL, TH, TL values, the coefficient factors in the scaling equation can be determined:
therefore, a liquid nitrogen calibration source box body above the antenna housing is removed, and the conventional foundation microwave radiometer can convert 0-level (voltage value) to 1-level (brightness temperature) in remote sensing data according to a calibration equation.
Example 2: internal calibration source calibration operation
1. Calibration of noise source port plane ((at this time, the waveguide switch is in a working state 1 (as shown in figure 1 below), ports 1 (feed source), 2 (directional coupler), ports 3 (load) and ports 4 (empty) are communicated, and TTL level of the noise source is in an on state at a high level)
As the above conventional radiometer calibration operations are consistent, the pitching stepping motor of the ground-based microwave radiometer is set to return to the original point, so that the antenna aperture surface points to the blackbody source inside the machine, the output voltage value Vnh when stable is read, and the temperature TH of the internal blackbody is read.
As the conventional radiometer calibration operation is consistent, the pitching stepping motor of the ground microwave radiometer rotates 50000 steps, the antenna opening face points to the random outer zone right above and soaks the liquid nitrogen calibration source, the output voltage value Vnl when the antenna opening face is stable is read, the environment temperature T0 and the earth surface atmospheric pressure are read, and the temperature is converted into the cold source black body temperature TL.
TL＝T0-0.00825×(1013.25-P0)
Then the noise source face calibration value is:
through the implementation, the calibration of the noise source is completed.
2. Load source aperture calibration
On the basis of noise source calibration, the waveguide switch is controlled to be in a working state 2 (shown in figure 1 below), 2 (directional coupler) ports, 3 (load), 1 (feed source) and 4 (empty) ports are communicated at the moment
Noise sources are controlled to be turned on through K2 (TTL is turned on when being high and is turned off when being low), stable voltage values Vr and Vnr of a receiver are respectively read, and the mouth surface equivalent brightness temperature of TR is obtained on the basis of a stable noise source calibration and calibration equation as shown in the following figure 4.
Through the implementation, the calibration of the load source is completed.
Example 3: real-time internal calibration operation in zenith atmosphere remote sensing
During actual remote sensing work, the antenna aperture surface receives an atmospheric radiation signal in the zenith direction, the pitching stepping motor of the foundation microwave radiometer rotates 50000 steps, the antenna aperture surface points to the zenith right above, and as shown in the following figure 5, remote sensing data are according to a calibration equation: and converting the voltage data V of 0 level into brightness temperature data T of one level.
In a remote sensing observation mode, the change period of data (namely digital integral weighting time) is flexibly set through interface software, and a corresponding upper computer forms a waveguide switch, a noise source and a control time sequence, as shown in fig. 6, so that one-time scaling operation is carried out before the next 0-level data is formed, and a coefficient factor of a corresponding scaling equation is formed.
In this way, the next remote sensing level 1 data is formed according to the new scaling equation T ═ K × V + b, and this is repeated.
Example 4: real-time internal calibration operation for satellite tracking remote sensing
When the satellite tracks the remote sensing working mode, presume the ephemeris parameter through the interface software, guide pitching stepping motor, position stepping motor to run automatically, make the aerial track the satellite and point to.
As in embodiment 3, the period of data change (which is much shorter than the time interval of antenna directional bit shift) is flexibly set by the interface software, and the corresponding upper computer forms the timing sequence for the waveguide switch and the noise source and control, as shown in fig. 6, so that a scaling operation is performed before the next 0-level data is formed, and the coefficient factor of the corresponding scaling equation is formed.
In this way, the next remote sensing 1-level data is formed according to the new scaling equation T ═ K × V + b, and in this way, repeatedly, in the star trail mode, the 1-level data file records the corresponding brightness temperature and the corresponding azimuth and pitch angle.
Example 5: real-time internal calibration source-noise source correction operation
In actual remote sensing operation, no matter in the atmospheric radiation remote sensing in the zenith direction in the above embodiment 3 or in the star trace mode, the ground-based microwave radiometer antenna opening surface observes a path radiation signal with cold air as a background according to program guidance.
The noise source correction period value (the minimum unit is 1 minute and is permanently larger than the data period, the maximum is infinity, the default typical value is 60 minutes, and the value is infinity, no correction is made) is set in the foundation microwave radiometer interface software. Starting from the remote sensing data, the upper computer of the ground-based microwave radiometer sends a return origin stepping instruction to the antenna driving motor according to the time calculated by the correction period value, guides the antenna aperture to point to the built-in calibration source TH, and carries out gap stop according to the time set in the calibration operation, as shown in fig. 7.
And respectively reading the voltage values Vnh and Vh of the receiver with or without noise under external calibration of the heat source, and determining the mouth equivalent brightness temperature correction value of the TN by combining the real-time calibration equation coefficient K value.
Tn*＝K×(Vnh-Vh)
The new Tn automatically completes the replacement of the old Tn value, and all relevant formulas. And when the staying gap is reached, the upper computer of the foundation microwave radiometer drives the motor instruction to return to the front remote sensing observation mode according to the set program. The operation is repeated, and the real-time correction operation of the internal calibration source-noise source is realized.
The first item in the embodiment is basically set by normal operation, and besides parallel selection of 3 and 4 modes, other items can be operated in series and set by ground-based radiometer programming interface software.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (6)
1. A foundation microwave radiometer four-point calibration method based on a waveguide switch is characterized by comprising the following steps:
step 1: four-point calibration;
step 2: the calibration operation is realized;
and step 3: and (4) real-time internal calibration during normal work.
2. The four-point calibration method for microwave radiometer based on waveguide switch as claimed in claim 1, wherein the four-point calibration configuration of step 1 comprises an antenna feed system consisting of antenna reflector-polarization splitter-K, V waveband feed source, a pitching scanning mechanism with stepper motor driving the main reflector to rotate, a receiving front end consisting of waveguide switch and directional coupling, a four-point calibration source consisting of liquid nitrogen cold calibration source, a thermal calibration source under normal temperature operation, a load under normal temperature operation, and a noise source, and a receiver.
3. The four-point calibration method for the waveguide switch-based ground-based microwave radiometer according to claim 2, wherein the calibration operation of step 2 is implemented by specifically including the following steps:
step 21: conventional black body external calibration, i.e., thermal calibration TH: an upper computer inside the foundation radiometer executes a program to guide the antenna pitching motor to point right below, and the main antenna aperture surface wave beam just shines into the thermal calibration source black body;
the control signal K1 makes the waveguide switch connect the feed source, the heat calibration source signal is transmitted to the directional coupler at the rear end through the antenna reflection surface and polarization separation, and finally transmitted to the radiometer receiver system, in addition, the control signal K2 does not excite the noise source to work at low level, and the detection voltage VH and the normal temperature TH of the corresponding receiver are directly read;
step 22: cold source external scaling, i.e. cold scaling source TL: the prepared external calibration source is placed above an antenna housing of the foundation radiometer, an internal upper computer executes a program to guide an antenna pitching motor to point right above, and the main antenna aperture surface wave beam just shines into a cold calibration source black body;
the same as step 21, the control signal K1 makes the waveguide switch connect the feed source, the cold source signal is transmitted to the directional coupler at the rear end through the antenna reflection surface and polarization separation, and finally transmitted to the radiometer receiver system, in addition, the control signal K2 low level does not excite the noise source to work, and the corresponding receiver detection voltage VL is directly read;
TL is calculated as follows:
TL＝T0-0.00825×(1013.25-P0)
wherein T0 is the environmental temperature at calibration time and has a unit of K, and P0 is the atmospheric pressure at calibration time and has a unit of Pa;
determining a K factor and a scaling equation through the external scaling operation of the steps 21 and 22:
T＝K×V+b
wherein T is the brightness temperature, K is a scale factor, and b is a constant factor;
step 23: external calibration of noise source, namely noise source TN: the noise source is controlled to be started through a K2 switch, and receiver voltage values Vnh and Vnl in the external calibration of the cold source and the heat source are respectively read, so that the equivalent bright temperature of the mouth surface of the TN is determined:
step 24: load source calibration, namely normal-temperature load TR: the signal of the antenna feed surface is cut off through K1 switch control, the antenna feed surface is connected with a broadband waveguide load, the noise source is controlled to be started through K2, and the voltage values Vr and Vnr of the receiver are respectively read, so that the equivalent brightness temperature of the mouth surface of the TR is determined:
and completing the four-point calibration operation through the four steps of operation.
4. The four-point calibration method for the waveguide switch-based ground-based microwave radiometer according to claim 3, wherein in step 22, the voltage signal output by the receiver in the microwave remote sensing is linearly related to the actual target remote sensing brightness temperature:
T＝K×V+b
wherein T is the bright temperature, K is a scale factor, b is a constant factor, and the accuracy of the T is determined by the accuracy of the target remote sensing bright temperature data;
the ground microwave radiometer equipment is highly accurate sensitive equipment, the sensitivity of the ground microwave radiometer equipment can reach below 1K, any small fluctuation can cause inaccuracy of measured data, and therefore errors and misjudgments are caused for subsequent inversion. The influence on the data accuracy can be obtained from the following three formulas:
wherein B is the bandwidth, τ is the integration time, TS is the system equivalent noise temperature, G0 is the system gain,
in order to be the inherent sensitivity of the system,
for the uncertainty caused by system gain fluctuations,
is the system sensitivity;
the first formula reflects the inherent sensitivity of the system, is determined by the overall noise temperature Ts of the system, and can calculate and verify through the mean square difference value of the sampling voltage while scaling and extracting the K factor outside the cold source and the heat source:
wherein the content of the first and second substances,
is the most sampled voltage mean square error;
the second term reflects the uncertainty caused by the system gain fluctuation, and the root mean square values of the second term and the second term reflect the data drift of the whole machine system, namely the system sensitivity.
5. The four-point calibration method for the waveguide switch-based ground-based microwave radiometer according to claim 4, wherein the real-time internal calibration in normal operation in step 3 specifically comprises the following steps:
step 31: factor correction during normal operation
In a normal working state, the antenna points to a zenith to receive an atmospheric radiation signal or a boundary layer mode and a tracking satellite mode, the antenna port surface does not carry out external calibration operation, but in order to ensure the accuracy in a data period, the internal calibration correction in one data period is realized by the waveguide switch and the noise source time sequence control:
and substituting the correction factors into the scaling equation to determine a new scaling equation: t ═ K × V + b, thereby implementing an internal scaling process;
step 32: noise source equivalent correction at gap time interval
The stability correction of an internal noise source is realized in a single-point external calibration mode, at a gap observed by remote sensing or at a boundary layer observation starting point, a program control program guides an antenna pitching motor to point to the right lower side, and a main antenna aperture surface wave beam just irradiates a thermal calibration source black body;
and controlling the noise source to be started through a K2 switch, and respectively reading voltage values Vnh and Vh of the noise receiver under the external calibration of the heat source, so as to determine the equivalent brightness temperature correction of the oral surface of TN:
Tn*＝K×(Vnh-Vh)。
6. the four-point calibration method for the waveguide switch-based ground-based microwave radiometer according to claim 5, wherein the four-point calibration operation of step 2 is performed by taking a half year as a cycle unit, the factor correction during normal operation of step 31 in step 3 is performed by taking a second as a cycle unit, and the noise source equivalent correction during the gap period of step 32 in step 3 is performed by taking a half hour as a cycle unit, so that the calibration accuracy and the reliability of the remote sensing data are ensured from long, short and medium periods.
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